Particle-size spectrometers

ABSTRACT

The present specification discloses and claims a particle-size spectrometer in which a stream comprising a fluid carrier medium containing particles under test, is conducted through a slender beam of light, the stray light appearing on the particles being conducted to a photomultiplier, and the concentration and/or size distribution of the particles is determined with reference to the output signals of the photomultiplier. The spectrometer has a chamber through which the stream and said beam of light, passes and intersects to form a centre of stray light. The chamber has an inner surface which surrounds the centre of stray light at least along an arc which extends up to the edge of the beam of light both in front of and behind the centre of stray light, the inner surface being arranged to receive and conduct the stray light to the photomultiplier which is connected to the chamber.

D United States Patent 1 1111 3,869,208 Lorenz 1 Mar. 4, 1975 PARTICLE-SIZE SPECTROMETERS OTHER PUBLICATIONS [75] Inventor: Gerhard Lorenz Gomngen Ruby Improvement for Lasers-Task 11; Atwood et 211.; Germany The Perkin-Elmer Corp; March 1963; pg. 10, 11, 12. [73] Assignee: Sartorius-Membranfilter GmbH,

Gottingen, Germany Primary Examiner-Ronald L. Wibert Assistant Examiner-V. P. McGraw [22] Flled' 1973 Attorney, Agent, or FirmEdmund M. Jaskiewicz [21] Appl. N0.: 322,394

[57] ABSTRACT Foreign Application Priority D313 The present specification discloses and claims a parti- Jan. 28, 1972 Germany 2204079 cle-size spectrometer in which a stream comprising a fluid carrier medium containing particles under test, is [52] U5. Cl 356/102, 250/227, 250/228, conducted hr gh a len m f ig h r y 356/103, 356/236 light appearing on the particles being conducted to a [51] Int. Cl. G01n 15/02, GOln 21/00 photomulti lier, and h n t i n n /0r ize dis- 158] Field of Search 356/102, 103, 104, 236; tribution of the particles is determined with reference 250/227, 228 to the output signals of the photomultiplier. The spectrometer has a chamber through which the stream and [56] References Cited said beam of light, passes and intersects to form a cen- UNITED STATES PATENTS tre of stray light. The chamber lhilS an inner SUXfZlCE I which surrounds the centre of stray light at least along gl ljf 356/103 an arc which extends up to the edge of the beam of 5 13 12/1963 light both in front of and behind the centre of stray 312313748 1/1966 Hassler et 3],, light, the inner surfacebeing arranged to receive and 3,535,531 10/1970 Neitze] conduct the stray light to the photomultiplier which is $1,646.35?! 2/1972 B01 et a1. connected to the chamber.

3.703.641 11/1972 Rosen 3.740.148 6/1973 Moroz et a1. 356/102 12 Clalms, 7 Drawmg Figures ll PARTICLE-SIZE SPECTROMETERS The present invention relates to a particle-size spectrometer for determining the size of particles, in which spectrometer a stream comprising fluid carrier medium containing particles under test, is conducted through a slender beam of light, the stray light appearing on the particles being conducted to a photomultiplier, and the concentration and/or size distribution of the particles being determined with reference to the output signals of the photomultiplier.

The stream may be a liquid, particularly water, or a gas, particularly air. Thus, impurities, particularly in waters and in the atmospheric air, can be determined by the particle-size spectrometer.

Particle-size spectrometers are known which utilize measurement of stray light for the purpose of analyzing the size of grain in an aerosol. However, all such spectrometers determine the stray light response either in a narrowly limited angular range about a specific direction of dispersion, for example 90 to the beam oflight, in an angular range between 15 and 165 to the beam of light, or about the scattering centre (i.e. the centre of stray light) in a solid angle which is substantially smaller than the full solid angle of 4 1r.

Accurate measurement of the particle concentration or particle size distribution, whether in a liquid or in a gas, is rendered difficult by virtue of the fact that the particles are generally of non-uniform configuration, do not absorb light, or absorb light to different degrees, and have different indices of refraction. The light striking a particle is available, with the exception of the portion' of light absorbed, for measurement as stray light. This stray light is reflected from the particle, bent, and, provided that the particle is not opaque, refracted. Owing to the different properties of the particles, the intensity of the stray light varies in different directions relative to the beam of light. The difficulty in manufacturing a satisfactory particle-size spectrometer resides in obtaining stray light responses which, as far as possible, are dependent only upon the size of the particle but which are largely independent of all other parameters such as the configuration, absorption capacity and refractive index of the particles.

A known particle-size spectrometer, which determines the intensity of the stray light at 90 to the direction of the beam of light, supplies results of sufficient accuracy only when measuring particles having a uniform refractive index, since, in this direction (i.e. at 90 to the beam of light), the stray light response is greatly dependent upon the refractive index and the absorption capacity of the particles in addition to being dependent upon the size of the particles.

Particle-size spectrometers, which use dispersion of light obliquely forwardly or in an angular range of 15 to 165 to the beam of light, are sufficiently accurate only when analyzing either particles without absorption capacity or particles having a substantially uniform absorption capacity. They are not suitable for accurate testing of natural aerosols or waters, since particles having different absorption capacities are present. Furthermore, in these angular ranges, the intensity of the stray light is not absolutely dependent upon the size of particle in the range of lam.

The same applies to stray light measurements in solid angles which are substantially smaller than the full solid angle.

An object of the present invention is to provide a particle-size spectrometer which is independent to the maximum possible extent, of the absorption capacity, the refractive index and the configuration of the particles, and which has high sensitivity and a satisfactory resolving power.

Attempts have already been made to fulfil these requirements by measuring the stray light response in only a very small angular range in the region of i 5 to the forward direction of the beam of light. However, difficulties were caused by the unfavourable signal/- noise ratio. The noise is caused by diffusion along the beam of light of the molecules in the air. Furthermore, an expensive system of screens which has to be adjusted extremely accurately, was required in order to separate the stray light signal from the primary beam of light. There is the risk of disadjustment of such devices during transport, and mass production is rendered difficult.

According to the present invention there is provided a particle-size spectrometer in which a stream comprising a carrier fluid medium containing particles under test, is conducted through a slender beam of light, a photomultiplier being arranged to receive stray light appearing on the particles, the concentration and/or size distribution of the particles being determined with reference to the output signals of the photomultiplier, said spectrometer comprising a chamber through which said stream containing the particles and said beam of light pass, a centre of stray light being formed where said stream and said beam of light intersect, said chamber having an inner surface which surrounds the centre of stray light at least along an are which extends up to the edge of the beam of light both in front of and behind the centre of stray light, said inner surface being arranged to receive and conduct the stray light to the photomultiplier which is connected to the chamber.

The inner surface can be virtually the entire inner surface of the chamber when measurement is made across the full solid angle, or can be a narrow, arcuate strip if measurement is to be made only within a plane angle about the centre of the stray light.

In any event, the maximum possible scattering angle range for the stray light response is picked up with the exception of a negligible residual amount, so that the stray light signal is rendered as far as possible independent of the absorption capacity, the refractive index and the configuration of the particles.

Preferably, the stream of particles and the beam of light intersect in the centre of the chamber substantially at right angles, so that the period of dwell of the particles in the beam of light, and thus the idle time of the system, is rendered as small as possible.

When stray light is measured over substantially the full solid angle, the chamber may be in the form of an Ulbricht sphere-type photometer, a light exit aperture which leads to the photomultiplier, being provided in the wall of the chamber, the cross-sectional area of the aperture being small relative to the surface area of the sphere. A screen is provided and arranged in the chamber to prevent the light exit aperture from receiving direct light from the centre of stray light.

The light emanating from the centre of the stray light is reflected many times in a diffused manner from the normally matt white inner surface of the Ulbricht sphere-type photometer, so that each unit area of the inner surface of the sphere is illuminated with virtually the same intensity. In a manner of speaking, the luminous density of the wall of the Ulbricht sphere arranged opposite to the light exit aperture is measured by the photomultiplier through the said light exit aperture which is associated therewith. The screen protects the photomultiplier from direct illumination from the centre of the stray light.

A particularly favourable signal/noise ratio is obtained by measuring over substantially the full solid angle. In a development of the invention, this ratio can be substantially further improved by arranging a tube within the chamber to surround the path of the beam of light from a light entry aperture in the wall of the chamber to the centre of stray light, the tube having a light absorbent interior surface and a reflective exterior surface. Diffusion to the air molecules in the path of the beam of light within the Ulbricht sphere, but in front of the scattering centre (i.e. centre of stray light), are then not detrimental to the results to be obtained, since the stray light from the air molecules remains within the tube. The exterior mirror-coating of the tube prevents the tube from absorbing a portion of the stray light to be measured.

On the other hand, if the measurement is made over a plane angle of approximately 180, and not over the full solid angle, this can be achieved in a further development of the present invention in that photoconductors leading from the photomultiplier extend into the chamber substantially in a common plane with the beam of light, the ends of the photoconductors within the chamber being directed towards the centre of stray light and terminating in an arcuate surface which surrounds the centre of stray light for approximately 180, the arcuate surface being narrow relative to the internal dimensions of the chamber and the inner wall of the chamber having a light absorbent surface.

In this instance, the photoconductors commonly serve as cross section transducers. Their receiving ends are arranged in an arc substantially in one plane, while their ends remote from the chamber which radiate the light, are bunched together such that they can conduct the light to the photocathode of the photomultiplier.

Measuring over a plane angle is particularly advantageous when measurements have to be undertaken on aerosols having widely varying refractive indices.

With advantage, reflectors may be arranged to extend from the ends of said arcuate surface, into the immediate vicinity of the light beam, which reflectors are so arranged to reflect light from the centre of stray light towards said arcuate surface i.e. towards the admission ends of the photoconductors.

The present invention will now be further described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 shows a section through a chamber in which a beam of light from an associated optical system intersects with an aerosol,

FIGS. 2 and 3 show further details of the optical system and the aerosol jet,

FIG. 4 shows a horizontal central section through a chamber, in the form of an Ulbricht sphere, for measuring over substantially the full solid angle,

FIG. 5 shows a horizontal central section through a chamber having photoconductors for measuring over a plane solid angle of approximately 180.

FIG. 6 shows the same chamber in a central section at right angles thereto, taken on the line AA in FIG. 5, and

FIG. 7 is a view of the admission surface of the photoconductors viewed from below in FIG. 5, all the screens having been omitted.

Referring to FIG. 1, a substantially punctiform source 2 of light projects light through an aperture in screen 6 via a condenser 4. The aperture in screen 6 is circular though alternatively it can be square. The source of light may be a superpressure mercury vapour lamp or an arc. Alternatively, a laser may be used from which the beam of light falls parallel onto a condenser which, has somewhat different dimensions to the condensor 4, this condenser focusing the light onto the aperture of the screen 6.

In the accompanying drawings the chamber 8 (FIG. 1) is shown only diagrammatically. Alternative constructions of chamber in accordance with the invention are illustrated in FIGS. 4, 5 and 6. In each chamber, an aerosol jet 10 is conducted through the centre of the chamber where it is to intersect the beam 12 of light.

Referring to FIG. 1, a cylindrical lens 14 reproduces the image of the aperture in the screen 6 as a straight line 16 at the location of the aerosol jet 10, as may be seen in FIG. 2, this line 16 extending at right angles to the drawing plane of FIG. 1 and intersecting the aerosol jet 10. The beam of light is focussed only at 18 in the plane of FIG. 2. This focusing is effected by a cylindrical lens 20 whose axis extends at right angles to the axis of the cylindrical lens 14 and which has a somewhat greater focal length than the cylindrical lens 14. Thus, an astigmatic reproduction of the aperture in the screen 6 is produced by the two cylindrical lenses l4 and 20. The two cylindrical lenses can be replaced by a spherical projecting lens with cylindrical grinding.

As is shown in FIG. 2, the aerosol jet 10 extends through the centre of the beam 12 of light which at this location, is still considerably wider than the aerosol jet. This ensures that all the particles entrained in the aerosol jet are illuminated. This arrangement is also advantageous when using a laser as the source of light, since the intensity of the laser light is at a maximum in the centre of the laser exit surface and decreases towards the edges in the manner of a Gaussian distribution. The aerosol jet then passes through the centre region of greatest intensity of the beam of light.

The beam originating from the cylindrical lenses is limited by a rectangular screen 22. A screen 24 arranged behind the screen 22 no longer serves to limit the beam but to receive and render harmless the diffraction light produced by the aperture in the screen 22. The cylindrical lenses 14 and 20 and the screens 22 and 24 are accommodated in a tube 26, the relative positions of the screens being adjustable by means which are not illustrated. Alternatively tube 26 can be replaced by a box, the tube or box connecting with part of a side wall of the chamber 8, in which an entry aperture 28 is provided for the beam of light; an exit aperture 29 being provided in the opposite wall of the chamber.

In practice a beam of light is preferably used which is substantially more slender than the beam of light illustrated in FIGS. 1 and 2, so that the entry aperture 28 and the exit aperture 29 of the chamber can be as small as possible, thus enabling measurement to be made over the largest possible angle.

The tube 26 is connected to the chamber in an airtight manner and its left hand end is sealed in an airtight manner by the cylindrical lens 14 which is cemented therein.

An absorption device 30, in the form of a Wood horn, for the emerging beam of light is connected to the 7 right hand side of the chamber 8 in an airtight manner over the exit aperture 29. The horn has a matt black coating on its inner wall and is designed such that the light is dissipated under continuous absorption after a few reflections. The tube 26, the chamber 8 and the Wood horn 30 have a common interior space which is sealed towards the outside and in which, as will be described later, a vacuum prevails.

The aerosol jet 10 is conducted near to the centre of the chamber 8 by means ofa glass capillary tube 32 i.e.

the inner capillary tube illustrated in FIG. 1. This inner capillary tube is surrounded by an outer capillary tube 33 which is also made from glass and through which filtered air is conducted. The air flow surrounds the aerosol fiow in the form of a laminar flow and focuses the aerosol flow so to speak, so that a thin aerosol flow 10 having a circular cross section in the centre of the chamber, i.e. the centre 35 of the stray light, intersects the beam 12 of light as is shown in FIG. 2.

A further glass capillary tube 37 extends into the chamber 8 from the wall of the chamber opposite the wall through which inner capillary tube 33 projects, capillary tube 37 being arranged coaxially with capillary tubes 32 and 33. The end of the capillary tube 37 (i.e. the top as viewed in FIG. 1) is widened towards the centre of the stray light by means of a funnel 38, and the outer end of the capillary tube 37 (i.e. the lower end in FIG. 1) is connected to a pump (not shown). The pump produces in the chamber 8 a pressure which is about 0.2 atmospheres below atmospheric pressure. Thus, aerosol and purified air is drawn in through the capillary tubes 32 and 33. Two passages 40, which are located diametrically opposite one another with respect to the capillary tubes, and which are directed towards the centre of the stray light, lead to the chamber wall and also feed filtered air. These additional air flows flush the chamber 8 free from particles and prevent widening of the aerosol jet when it emerges from the capillary tube 33.

In addition to controlling the aerosol jet, the suction also removes other impurities from the air within the chamber 8, the tube 26 and the Wood horn 30. As described above, all three parts have a common interior space which is closed on the left only by the cylindrical lens l4, so that a requirement for special windows, made from glass for example, is avoided at the apertures 28 and 29 of the chamber and thus the disadvantage of dispersion of light at these windows is obviated.

The aerosol is rarified to an extent where two or more particles virtually never pass through the beam of light simultaneously. Thus, each individual particle produces a flash of stray light which is measured individually. If a particularly high throughput of aerosol is required, the bottom ends of the capillary tubes 32 and 33 can be chamfered in the direction of the beam of light, thus resulting in an elongate aerosol flow 10a (shown in FIG. 3) which, in the same manner as the aerosol flow in FIG. 2, passes only through the central region of the beam of light.

Shading of the stray light is substantially avoided by virtue of the fact that the capillary tubes 32, 33 and 37 are made from glass, which capillary tubes 32 and 33 are adjustable relative to the beam of light.

For the purpose of measuring over substantially the full solid angle, the chamber 8a shown in FIG. 4 is in the form of an Ulbricht sphere. It has a spherical, matt white inner surface 42 having an internal diameter of,

for example 6 cm or more, and an exit opening 44 through which the stray light is conducted to a photomultiplier 46. The cross section of the opening 44 should be small relative to the entire inner surface 42. The centre 35 of stray light i.e. the region where the aerosol and the light beam intersect, is shaded from the opening 44 by a screen 48, so that no direct light can enter the photomultiplier from the centre of stray light. The screen 48 is held by an externally polished rod (not illustrated).

A tube 49, which has a reflective outer surface and a matt black inside surface, is connected to the light entry opening 28 (FIG. 4) and extends towards the centre 35 of the stray light, so that the tube surrounds the beam 12 of light at a distance therefrom. This prevents falsification of measurement by dispersion 'of light resulting from the interaction of the light beam with particles which are still present in the interior space of the chamber despite the filtering of the air, prior to its interaction with the aerosol jet 10.

In a conventional manner, the photomultiplier has a photocathode from which the received light releases electrons which are then fed to a secondary electron multiplier for amplification.

As in FIGS. 5 and 6, details of the optical device and the inlets and outlets for the aerosol and the air have been omitted in FIG. 4 for the sake of clarity. These parts and arrangements are shown diagrammatically in FIGS. 1 to 3.

As already mentioned, each time a particle passes through the beam of light, a flash. of light occurs which lights up the inner surface 42 of the Ulbricht sphere. The brightness of the inner surface is then measured by the photomultiplier.

If the particle-size distribution in the aerosol is to be established, the output of the photomultiplier is connected to an amplitude analyzer having a plurality of outputs for different ranges of amplitude and thus different ranges of particle size. A counter is connected to each of the outputs. The particle-size distribution can be ascertained by analyzing the counting results.

On the other hand, if the particle concentration in the aerosol is to be determined, the output signal of the photomultiplier is fed to a pulse integrating circuit whose output is connected to an analog indicator. Thus, an indication of the total volume of the particles per cubic metre of air is obtained. If the average specific weight is known, the total weight in mg/m is calculated therefrom. Furthermore, if the size distribution of the particles is known from the above-mentioned evaluation, the number of particles per cubic metre can be obtained.

In the embodiment illustrated. in FIGS. 5 to 7, the output signals of the photomultiplier 46 are evaluated in the same manner. However, measurement is made over a plane angle of about and not over the full solid angle as in the embodiment in FIG. 4. In this instance, the chamber 8b is of cubic configuration, though it may alternatively be of parallelepiped, spherical configuration. The chamber 8b has a matt black, i.e. lightabsorbent, inner surface 50 and photoconductors 52 extend into the chamber and together form a cross section transducer. The photoconductors extend from the chamber 8b to form a bundle 52.1 of circular cross section which conducts the light received to the photomultiplier 46. The ends of the light conductors in the chamber 8b are arranged to form a common circularcylindrical entry surface 54 whose centre of curvature lies on the centre of the aligned aerosol jet 10, this surface 54 having only a very small width compared to its radius, as is shown in FIGS. 6 and 7. The portions of the photo-conductors adjacent the entry surface 54 are surrounded on both sides by a layer 55 of plastics material compound which serves only to support the photoconductors. As is shown by broken lines (FIGS. and 6) the individual photoconductors are guided such that their entry ends extend radially of the centre 35 of stray light.

The size of the chamber 8b is such that the light, scattered from its walls, is of negligible intensity compared with the light received by the photoconductors.

For reasons of adjustment, it is disadvantageous to allow the ends of the curved entry surface 54 (as viewed in FIG. 5) to extend directly up to the beam 12 of light. Two sharp-edged adjustable reflectors 56 and 57 are secured to the ends of the entry surface 54 and are adjusted such that they extend as near as possible to the beam of light and reflect stray light towards the entry surface 54.

Diffraction light appears on the reflector 56 and must be prevented from falling onto the entry surface 54. For this purpose, a screen 60 is arranged in the plane of the aerosol jet and radially of the entry surface 54. The surface of the screen 60 facing the reflector 56 is matt black. By virtue of the radial arrangement of this screen, the shadow angle of the radiation emanating from the centre of stray light is reduced to a minimum. As is illustrated, the screen 60 extends up to the entry surface 54 though this is not necessary. With the embodiment illustrated, the reflector 56 has to be inclined such that all the stray light falling thereon is directed onto the entry surface portion 54.1 between the screen 60 and the reflector 56. A screen 62 serves to define the other side of the beam 12 of light.

In the embodiments illustrated in FIG. 4 on the one hand and FIGS. 5 to 7 on the other hand, the respective surfaces 42 and 54, 56, 57 absorbing the stray light extend to the light beam up to less than 8, particularly less than 3, measured from the centre of stray light and the axis of the light beam.

The described arrangements can also be used for measuring the concentration and size distribution of particles in gases other than air. Furthermore, with only slight modifications, the arrangements can be used for taking measurements in fluids, particularly water. In this case, the fluid containing the particles is conducted through the inner capillary tube 32, and pure fluid, not containing particles, is conducted through the outer capillary tube 33. The jet of liquid emerging from the outer capillary tube 33 crosses the beam of light and is drawn off by the capillary tube 37. In orderto avoid the total reflection in the centre of stray light on the jet of fluid, the entire chamber may be flushed through the channels 40 with fluid, free from particles. The fluid containing particles then forms a separate flow path within the fluid free from particles, entering the capillary tube 37 from the environment.

The opening 28 is closed in a watertight manner by means of a plane-parallel glass plate.

I claim:

1. A particle-size spectrometer in which a stream comprising a carrier fluid medium containing particles under test, is conducted through a slender beam of light, a photomultiplier being arranged to receive scattered light appearing on the particles, the concentration and/or size distribution of the particles being determined with reference to the output signals of the photomultiplier, said spectrometer comprising a chamber through which said stream containing the particles and said beam of light pass, said chamber comprising an Ulbricht sphere-type photometer for measuring stray light over substantially the full solid angle, a center of stray light being formed where said stream and said beam of light intersect, said chamber having an inner surface which surrounds the center of stray light at least along an arc which extends up to the edge of the beam of light both in front of and behind the center of stray light, said inner surface having a light exit aperture therein to receive and conduct the stray light to the photomultiplier which is connected to the chamber, the cross-sectional area of the aperture being small with respect to the surface area of the sphere, and a screen positioned in the chamber to shade the aperture from receiving direct light from the center of stray light.

2. A particle-size spectrometer according to claim 1, wherein the stream containing the particles and the light beam intersect in the centre of the chamber substantially at right angles.

3. A particle-size spectrometer according to claim 1 wherein a light entry aperture is provided in the wall of the chamber, a tube within the chamber to surround the path of the beam of light, said tube extending from the light entry aperture to the center of stray light, and having a light absorbent interior surface and a reflective exterior surface.

4. A particle-size spectrometer according to claim 1, wherein a first translucent capillary tube is connected to a source of the stream and is arranged to conduct the stream to the center of the chamber, a second translucent capillary tube being arranged coaxially with and surrounding the first capillary tube at a distance therefrom, this second translucent capillary tube extending further toward the center of stray light than said first capillary tube and tapering continuously toward the latter, the annular space between the two capillary tubes serving to conduct particle free carrier fluid of the stream.

5. A particle-size spectrometer according to claim 1, wherein the ends of the first and second capillary tubes directed towards the centre of the chamber have an elongate cross-section for relatively high stream throughputs, the longitudinal direction of which crosssection extends parallel to the direction of the light beam.

6. A particle-size spectrometer according to claim 1,

wherein the interior of the chamber is filled with the same fluid though particle-free, as the stream containing the particles.

7. A particle size spectrometer having a chamber, there being a light entry aperture in the wall of the chamber, light conductors leading from a photomultiplier and extending into the chamber, said light conductors being arranged to receive scattered light over a plane angle of substantially 180 essentially in a common plane with the light beam, the ends of the light conductors within the chamber being directed toward the center of scattered light, an arcuate surface formed by said ends of the light conductors surrounding the center of scattered light for nearly 180 wherein a first adjustable reflector is arranged to extend from the end of the arcuate surface near the light entry aperture and defines a screen forming one of the edges of the beam such that a portion of the light is diffracted at the edge of the reflector, a second adjustable reflector extending from the other end of the arcuate surface into the immediate vicinity of the light beam, both reflectors being so arranged as to reflect light from the scattering center toward said arcuate surface, a screen within the chamber, said screen being arranged radially of said scattering center, the side of the screen facing said first reflector having a light absorbent surface for the purpose of absorbing said light diffracted at the edge of said first reflector.

8. A particle-size spectrometer according to claim 7, wherein a first translucent capillary tube is connected to a source of the stream and is arranged to conduct the stream to the center of the chamber, a second translucent capillary tube being arranged coaxially with and surrounding the first capillary tube at a distance therefrom, this second translucent capillary tube extending further toward the center of stray light than said first capillary tube and tapering continuously toward the latter, the annular space between the two capillary tubes serving to conduct particle free carrier fluid of the stream.

9. A particle-size spectrometer according to claim 7, wherein the ends of the first and second capillary tubes directed toward the center of the chamber have an elongate cross-section for relatively high stream throughputs, the longitudinal direction of which crosssection extends parallel to the direction of the light beam.

10. A particle-size spectrometer according to claim 7, wherein the interior of the chamber is filled with the same fluid though particle-free, as the stream containing the particles.

11. A particle-size spectrometer according to claim 7, wherein the arcuate surface is narrow relative to the internal dimension of the chamber.

12. A particle-size spectrometer according to claim 7, wherein the inner wall of said chamber has a light absorbent surface. 

1. A particle-size spectrometer in which a stream comprising a carrier fluid medium containing particles under test, is conducted through a slender beam of light, a photomultiplier being arranged to receive scattered light appearing on the particles, the concentration and/or size distribution of the particles being determined with reference to the output signals of the photomultiplier, said spectrometer comprising a chamber through which said stream containing the particles and said beam of light pass, said chamber comprising an Ulbricht sphere-type photometer for measuring stray light over substantially the full solid angle, a center of stray light being formed where said stream and said beam of light intersect, said chamber having an inner surface which surrounds the center of stray light at least along an arc which extends up to the edge of the beam of light both in front of and behind the center of stray light, said inner surface having a light exit aperture therein to receive and conduct the stray light to the photomultiplier which is connected to the chamber, the cross-sectional area of the aperture being small with respect to the surface area of the sphere, and a screen positioned in the chamber to shade the aperture from receiving direct light from the center of stray light.
 2. A particle-size spectrometer according to claim 1, wherein the stream containing the particles and the light beam intersect in the centre of the chamber substantially at right angles.
 3. A particle-size spectrometer accordIng to claim 1 wherein a light entry aperture is provided in the wall of the chamber, a tube within the chamber to surround the path of the beam of light, said tube extending from the light entry aperture to the center of stray light, and having a light absorbent interior surface and a reflective exterior surface.
 4. A particle-size spectrometer according to claim 1, wherein a first translucent capillary tube is connected to a source of the stream and is arranged to conduct the stream to the center of the chamber, a second translucent capillary tube being arranged coaxially with and surrounding the first capillary tube at a distance therefrom, this second translucent capillary tube extending further toward the center of stray light than said first capillary tube and tapering continuously toward the latter, the annular space between the two capillary tubes serving to conduct particle free carrier fluid of the stream.
 5. A particle-size spectrometer according to claim 1, wherein the ends of the first and second capillary tubes directed towards the centre of the chamber have an elongate cross-section for relatively high stream throughputs, the longitudinal direction of which cross-section extends parallel to the direction of the light beam.
 6. A particle-size spectrometer according to claim 1, wherein the interior of the chamber is filled with the same fluid though particle-free, as the stream containing the particles.
 7. A particle size spectrometer having a chamber, there being a light entry aperture in the wall of the chamber, light conductors leading from a photomultiplier and extending into the chamber, said light conductors being arranged to receive scattered light over a plane angle of substantially 180* essentially in a common plane with the light beam, the ends of the light conductors within the chamber being directed toward the center of scattered light, an arcuate surface formed by said ends of the light conductors surrounding the center of scattered light for nearly 180* wherein a first adjustable reflector is arranged to extend from the end of the arcuate surface near the light entry aperture and defines a screen forming one of the edges of the beam such that a portion of the light is diffracted at the edge of the reflector, a second adjustable reflector extending from the other end of the arcuate surface into the immediate vicinity of the light beam, both reflectors being so arranged as to reflect light from the scattering center toward said arcuate surface, a screen within the chamber, said screen being arranged radially of said scattering center, the side of the screen facing said first reflector having a light absorbent surface for the purpose of absorbing said light diffracted at the edge of said first reflector.
 8. A particle-size spectrometer according to claim 7, wherein a first translucent capillary tube is connected to a source of the stream and is arranged to conduct the stream to the center of the chamber, a second translucent capillary tube being arranged coaxially with and surrounding the first capillary tube at a distance therefrom, this second translucent capillary tube extending further toward the center of stray light than said first capillary tube and tapering continuously toward the latter, the annular space between the two capillary tubes serving to conduct particle free carrier fluid of the stream.
 9. A particle-size spectrometer according to claim 7, wherein the ends of the first and second capillary tubes directed toward the center of the chamber have an elongate cross-section for relatively high stream throughputs, the longitudinal direction of which cross-section extends parallel to the direction of the light beam.
 10. A particle-size spectrometer according to claim 7, wherein the interior of the chamber is filled with the same fluid though particle-free, as the stream containing the particles.
 11. A particle-size spectrometer according to claim 7, wherein the arcuate surface is narrow relativE to the internal dimension of the chamber.
 12. A particle-size spectrometer according to claim 7, wherein the inner wall of said chamber has a light-absorbent surface. 